At swap picked up a small 3hp Briggs with cast iron water pump on it. $20. Kept hoping the shaft wasn't gonna be wierd for the pump. Removed pump, gave to my buddy and shaft is common 5/8" and will work on my Mini-bike I'm thinking. Had old gas in it, got lots of good spark. Didn't seem to fire on old gas, so set aside in truck. Was tilted some in truck and when got home found oil under it, oops! Seemed to be on flywheel side. Rod and piston still working. So today I took off the shrould and had tons of junk in it, all oily. Pulled flywheel, expecting crank seal. Seal looked OK, later found tiny hole in bottom of case. Poked at it a little, got bigger. Aluminum looked kinda funny there in area, so poked around and another hole pushed thru, then another! Hmmmm! I'm really thinking case is just ate away from something in the crap that was jambed in that area. Ever hear of such a thing, chemicals eating away at case? Guess I will be pulling it apart to make sure no rod pieces or other stuff broken that might have flown around inside. Really don't look like that type of damage. I see another spot in group of holes that is thin and looks like it could be just ate away there also. SO, think new case time, or JB Weld would hold? OR, throw the engine and move on? Not sure if cases can be heli-arced and worth the cost? This is a new one to me, just my luck lately!

I have a Clinton engine that the aluminum case has broken down over the years so that you can rub it between two fingers and it just crumbles. Was a sweet running engine before it dissolved while running. Wasn't visually corroded or oxidized at all.

Salt, lye, acids will all cause cast aluminum to corrode like that. They will actually crumble like stale croutons if bad enough.
GL, after you get the bad areas found, wash it with baking soda and water. See if you get any fizzing. If not, it was probably a base like lye.

Plumbers used to use pure lye as a drain cleaner many moons ago and my experience is it is HARD on metals, especially the softer ones. Use water to clean the case up as best you can to dilute whatever was eating it (if any left) and fill it up with JB weld. You're only out $20 and whatever you end up with in the JB. Think of it as an experiment.

Was picking up in garage to make room for a project and quickly tore apart the engine today. Rod looked good, has a Darn PLASTIC camshaft! The holes don't look like from fragments exploding or such then. Looks cleaner inside on the metal in area. I think whatever that mess was under flywheel and cover area that was wet and I dug out was holding something in it, that ate the area out from outside. Will have to get the case cleaned up better, try to take a pix then. Think I will blast the area and see what happens, has to clean up better for sure for patch to hold, if I DO that, which is what it is looking like I will do.

Here's some pix of my block after I blasted that area. You can see it looks weak in big area on outside, right where the fiber junk was all mashed into itl One inside shot, holes look clean, not knocked outward, so not from rod or such inside. Think JB weld will hold? Even the lip that outer cover lines up on is kinda gone, started into one mount bolt for cover.

I've seen the same thing on our Honda 350 Foreman we used to have. But it was the differential that got a hole eaten through the aluminum. Manure between the steel drag shield & the aluminum housing apparently set up a form of electrolysis. I dug out all the corroded aluminum, then JB Welded it. Held fine, and I left off the shield.

Well, tried the JB today and had some trouble. That stuff OOZ's like the "Blob" in the old movie. I figured some would ooz into the inside, BUT, it never quit oozing. Then put a piece of cardboard on there to block it, later when I moved the engine to check, the whole blob with the patch came out together. Everytime I tried to move engine to diff position, stuff just oozed that way and ran off still. Tonight it is finally set up, but kinda caved in where the holes were, but not broken thru, stuff in places I really was trying to scrape off in each move and will probly have to grind or scrape away again! Never had such an issue with JB before. They have the quick set kind, which I heard wasn't so good, maybe it would have better here? Anyway, question now is CAN I do second coat over the orig patch and have it hold still? It will need more, won't trust what is in the holes as being enough now. Hopefully a smaller amount will be needed, more to fill those holes than over the entire area again. Think inside should be good, but outside needs more as noted. Am I good to double fill?